The present invention refers to a hermetic compressor for a refrigeration system and, more specifically, to a system for assembling a shaft in an electric motor rotor, said rotor comprising a metallic core carrying magnets.
Motors provided with a rotor carrying permanent magnets have been largely used in industrial applications and in domestic equipments. Nowadays, such motors are also used in hermetic compressors of refrigeration appliances.
The rotor carrying permanent magnets is rigidly mounted onto a portion of an eccentric shaft of a compressor, in order to resist to the transmission of torque generated by the motor to the eccentric shaft, to the efforts due to impacts produced during transportation and also to thermal variations which are inherent in the operation of the compressor (heating and cooling of the compressor). Moreover, this assembly is so effected as to avoid the occurrence of gaps between the rotor and the shaft, avoiding relative movements to appear therebetween.
In these rotors, the magnets are affixed to the rotor core by means of a technique which imparts to the rotor a mechanical structure required for the operation of the compressor. The retention of the magnets to the rotor core may be achieved by processes, such as chemical retention (adhesion) of the magnets to the rotor core and partially or completely encapsulating the magnets on the rotor core, for example with a thermoretractible or thermoinjectable film or by using a metallic cover.
One of the known techniques for affixing the rotor with permanent magnets onto the compressor shaft consists in heating the rotor, until dilatation of its internal diameter has been achieved, in order to allow the shaft to be freely introduced into said rotor.
The attachment of the shaft to the rotor, after said shaft has been introduced into said rotor, occurs during the cooling and consequent contraction of the rotor, which, by reducing the diameter of the motor hole, eliminates the assembly gap and adjusts the rotor to the shaft, guaranteeing the attrition which affixes these parts together.
A deficiency of using the technique of heating the rotor to be assembled by interference to the compressor shaft is that, in the case of magnets glued around the rotor core, it may occur degradation in the resistance properties of the adhesive used during the heating process, causing the release of said magnets immediately or during the operation of the compressor.
In the case of rotors encapsulated with the thermoretractible or thermoinjectable material, the same effect is possible. In the rotors encapsulated by a metallic cover, cracks may appear in the magnets, provoked by tensions resulting from the difference between the thermal dilatation coefficients of the materials of the magnet and metallic cover.
Another disadvantage is the need for having a high power oven or heating device for the rotor, as a function of its thermal inertia and the short time interval it remains in the assembly line.
Another known technique for assembling the rotor to the shaft uses mechanical interference between these parts and consists in inserting the shaft into the rotor when the difference between their temperatures is small or even null. In this case, the interference coupling is obtained by applying a high axial assembly load in both components (rotor and shaft).
In the assembly with no heating or with little rotor heating, the shaft is introduced with a great effort into the central hole of the rotor, which usually deforms resilient components or fragile parts of the assembly and allows burrs or fragments to be produced due to the high attrition between the rotor and the shaft. Another disadvantage is the high cost for producing the components, due to the finishing tolerances required for the compressor shaft and for the internal diameter of the rotor, as a form of limiting the assembly loads. Even establishing precise dimensions for both the rotor and shaft, a great difficulty remains in obtaining a relative axial positioning with the desired precision therebetween.
Another known technique, in which the shaft is cooled, has as disadvantage the high manufacture cost of the components, due to the finishing tolerances required for the compressor shaft and for the internal diameter of the rotor. Another disadvantage of this technique is the short time interval available for the rotor to be mounted to the shaft, before the temperature of said shaft raises up to a value in which the operation is impossible to be executed.
The attachment between the rotor with permanent magnets and the shaft may also be achieved by using adhesive or chemical retention, in which an adhesive is applied into a gap, which is previously determined between the internal diameter of the rotor and the external diameter of the compressor shaft. The retention may be also obtained by a welding process, with or without deposition of material between the shaft and the rotor.
The technique in which the shaft is chemically retained to the rotor has the inconvenience that the adhesive employed may contaminate the gap existing between the shaft and a supporting bearing thereof provided in the compressor, locking these parts together and impairing the perfect sliding therebetween and even avoiding the operation of the compressor. Another disadvantage of this technique is that the compressor shaft may be contaminated with oil, before the adhesive is applied to the surface, which causes a degradation in the properties of chemical retention.
In the case of welding, with or without deposition of material, the disadvantages consist in the little space available for the application of welding, generation of sparks, arcs and/or gases, and rotor heating, which may cause degradation of the adhesive used to retain the magnets to the core or to affix the thermoretractible or thermoinjectable cover.
Thus, it is an objective of the present invention to provide a hermetic compressor having a rotor with permanent magnets with a system, which allows assembling the motor shaft of the compressor into the motor rotor with permanent magnets, without submitting fragile or heat susceptible components to heat or to high efforts which may cause damages to said parts, minimizing losses and leading to an economical and industrially viable result.
A further objective of the present invention is to provide a hermetic compressor such as mentioned above, which allows the shaft to be affixed to the rotor, without requiring said shaft to be previously cooled and assuring a correct and accurate relative positioning between said parts.
A specific objective of the present invention is to provide a system and a process for assembling the shaft to the rotor, which allows said assembly to be achieved by one of the interference and gluing techniques.
These and other objectives are attained by a hermetic compressor for a refrigeration system, having an electric motor including a rotor, which is rotatively and axially affixed around an axial portion of the shaft of the electric motor, said compressor comprising a tubular bushing, which is seated onto and rotatively and axially affixed around the shaft, in order to involve at least said axial portion of the latter inside the rotor. The assembly of the shaft inside the rotor is achieved by first assembling and affixing the tubular bushing around the axial portion of the shaft and then assembling and affixing the shaft carrying the tubular bushing into a central hole provided in the rotor.